1. Field of the Invention
The present invention relates generally to tapered optical fibers and is particularly applicable to optical fibers for use in optical couplers that have low values of nonadiabatic-taper-induced loss.
2. Technical Background
Tapered or heat-treated optical fibers are utilized in fiber pigtails, optical couplers, splicing, connectors and other optical components. As the preformance requirements for the optical components become ever more stringent, the need to eliminate excess loss becomes more critical. One contributor to excess loss is non-adiabatic taper-induced loss.
In the tapered regions of fibers the fundamental mode is continuously changing shape to accommodate the changing local index profile. If the rate of change in geometry is too great, the fundamental mode can be coupled to the higher order modes. This mechanism is referred to as a non-adiabatic modal coupling.
When the optical fiber is heated, the dopant materials diffuse, changing the local refractive index profile. This occurs, for example, for example, during the splicing process, or during the manufacture of optical coupler devices. If the rate of change of refractive index (along the length of the fiber) is relatively large, the fundamental mode can be coupled to the higher order modes, resulting in a nonadiabatic index coupling.
The relative refractive index (Δ%) profile (idealized) of a typical telecommunication fiber is illustrated schematically in FIG. 1. FIG. 2 illustrates schematically a fiber profile with a depressed index of refraction in the region between the fiber core and the cladding. When tapered, these types of optical fibers exhibit a relatively large amount of non-adiabatic taper-induced loss. Therefore, optical components that include these optical fibers also suffer from non-adiabatic taper-induced loss. For example, an optical coupler can be fabricated by bringing together two optical fibers and then tapering and fusing them together with an appropriate heat source. However, as the fiber core diameter (in the coupling region) decreases, as compared with the core diameter at the end of each fiber, the coupler exhibits larger amounts of non-adiabatic taper induced loss.
U.S. Pat. No. 5,412,725 discloses a fiber coupler exhibiting a low non-adiabatic-taper induced loss. The fiber in this coupler has a refractive index profile illustrated in FIG. 3. More specifically, the refractive index profile illustrated in FIG. 3 has a refractive index pedestal characterized by an index of refraction that is intermediate the index n1 of the core and refractive index n2 of the cladding. However, as disclosed in col. 6, lines 8 of this reference, the optimal refractive values for the pedestal and pedestal radius were derived without considering such factors as mode field diameter and cutoff wavelength. The presence of a pedestal in the refractive index profile of a fiber results in a different mode field diameter than would be exhibited by a fiber without the pedestal.